Pin microshift build for openshift-4.18 rc.6 (#6018)

Created by job run https://art-jenkins.apps.prod-stable-spoke1-dc-iad2.itup.redhat.com/job/aos-cd-builds/job/build%252Fbuild-microshift/1646

Co-authored-by: AOS Automation Release Team <noreply@redhat.com>

releases.yml

@@ -16,7 +16,12 @@ releases:
         upgrades: 4.17.11,4.17.12,4.17.13,4.17.14,4.18.0-ec.3,4.18.0-ec.4,4.18.0-rc.0,4.18.0-rc.1,4.18.0-rc.2,4.18.0-rc.3,4.18.0-rc.4,4.18.0-rc.5
       members:
         images: []
-        rpms: []
+        rpms:
+          - distgit_key: microshift
+            why: Pin microshift to assembly
+            metadata:
+              is:
+                el9: microshift-4.18.0~rc.6-202501231641.p0.g2018187.assembly.rc.6.el9
       rhcos:
         rhel-coreos:
           images: